A swarm of earthquakes that began Sunday morning in southern California, and was still shaking Monday afternoon, registered more than 300 small to moderate earthquakes that could be felt from San Diego to southwest Arizona.

The type of activity that we're seeing could possibly continue for several hours or even days, said U.S. Geological Survey geophysicist Robert Graves.

Seismologists say swarms of earthquakes are not all that unusual, but the one currently underway in Southern California is different.

The quakes were so close together there, that at times it must have felt like one long earthquake for people down there, said Washington state seismologist John Vidale.

Instructors at the UW seismic lab are glued to the screens.

Most of the reason they're watching is it's deforming fast, said Vidale. The tectonic plates are really grinding up Southern California.

Seismologists say in the last 20 years, there have been swarms in Spokane and Klamath Falls areas that are similar to what is happening in Southern California.

The series of quakes has also raised awareness in the Northwest where contractors like A-FFIX say they get many calls for home retrofitting whenever earthquakes make headlines.

A-FFIX manager Bruce Schoonmaker said the California situation is ideal because nobody is getting seriously hurt, but awareness is spreading.

USGS seismologist Lucy Jones said earthquake swarms in California are characteristic of the region, known as the Brawley Seismic Zone.

The area sees lots of events at once, with many close to the largest magnitude, rather than one main shock with several much smaller aftershocks, Jones said.

Sunday's quake cluster occurred in what scientists call a transition zone between the Imperial and San Andreas faults, so they weren't assigning the earthquakes to either fault, said Graves.

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